As a conventional electromagnetically driven valve, for example, U.S. Pat. No. 6,467,441 specification discloses an electromagnetic actuator actuating valves of an internal combustion engine as a result of cooperation of electromagnetic force and a spring. The electromagnetic actuator disclosed in the specification is called a rotary drive type, and includes a valve having a stem and an oscillating arm having a first end hinged on a support frame and a second end in abutment on the upper end of the stem.
An electromagnet consisting of a core and a coil wound around the core is arranged above and below the oscillating arm. The electromagnetic actuator further includes a torsion bar provided at the first end of the oscillating arm and moving the valve toward a position of maximum opening and a helical spring arranged on an outer circumference of the stem and moving the valve toward a closed position. The oscillating arm oscillates using the first end as a fulcrum, in a manner alternately attracted to and contacted with the cores of the electromagnets arranged above and below the same.
Japanese Patent Laying-Open No. 09-133010 discloses an apparatus for electromagnetically driving a valve called a parallel drive type, which aims at power saving and improvement in response. The apparatus for electromagnetically driving a valve disclosed in this publication includes a valve element having a valve shaft fixed. The valve shaft is connected to a ring-shaped plunger through a plunger holder. A first electromagnetic coil and a first core are disposed above the plunger, and a second electromagnetic coil and a second core are disposed below the same. An upper spring moving the plunger downward is disposed further above the first electromagnetic coil and the first core, and a lower spring moving the plunger upward is disposed further below the second electromagnetic coil and the second core.
In the parallel drive type apparatus, an electromagnet constituted of an electromagnetic coil and a core and applying electromagnetic force to the plunger, and an upper spring and a lower spring applying elastic force to the valve shaft are arranged in series in a direction in which the valve shaft extends. According to such a structure, the electromagnetic force and the elastic force directly act on the valve shaft, so as to cause the valve element to carry out reciprocating motion.
According to the electromagnetic actuator disclosed in U.S. Pat. No. 6,467,441 specification, the oscillating arm comes in contact with an entire end surface of the core of the electromagnet when the oscillating arm is attracted to and contacts with the electromagnet. Accordingly, sound produced by collision between the oscillating arm and the electromagnet is great, and quietness when the electromagnetic actuator is driven is not satisfactory. In addition, as large repeated load is imposed on the oscillating arm moving at a high speed, the arm tends to be fractured in the vicinity of the first end. In order to solve this problem, it is possible to improve strength by increasing an overall thickness of the oscillating arm. In this case, however, a weight of the oscillating arm becomes too large, resulting in increase in energy loss.
In addition, according to the electromagnetic actuator disclosed in U.S. Pat. No. 6,467,441 specification, collision between the oscillating arm and the core of the electromagnet is repeated, which results in a problem of durability of the electromagnet. If the core is broken, the electromagnet should be exchanged, which impairs maintenance performance of the electromagnetic actuator. Such a problem also occurs in the apparatus for electromagnetically driving a valve disclosed in Japanese Patent Laying-Open No. 09-133010.